When converting with calibre, all line-height properties are written like this :
line-height: 1.2;
Shouldn't it be
line-height: 1.2em;

No. The first form is unitless. The second form has units of em. If you use a unitless line-height definition, all descendants get the line height specified as a ratio of their units. If you use units, all descendants get the line height based on the parent line height. Unitless is the preferred way to define line heights.